Pin for brazing to a substrate

Abstract

 A pin (6) is attached by braze material (3) to a solid state module (2) for making connection between the module and other parts of a metallurgical system. The end of the pin that is brazed to the module has a sloping head of conical shape (7), so that gas bubbles can readily escape during brazing, thus reducing voids in the braze.

Description

[0001] This invention relates to pins for brazing to a substrate. Such a pin is brazed to a solid state module to provide connection between the module and other parts of an electrical system.

[0002] In the manufacture of solid state components, pins that are typically made of Kovar are brazed to a substrate in order to provide electrical and/or mechanical contact between the substrate and other parts of an electrical system. The brazed joint between the pin and the substrate is a potential source of mechanical and electrical failure. It has been found that the potential for mechanical failure of the brazed joint is increased by the presence of voids in the braze material between the end of the pin and the surface of the module. Such voids result from gases within the molten braze that were unable to escape before the braze solidified. The voids form an easy path for propagation of cracks within the braze leading to failure of the brazed joint.

[0003] The invention seeks to provide a pin which reduces the formation of voids in the braze, thereby providing an improved brazed joint.

[0004] Accordingly, the invention is characterised in that the one end of the pin, which is to be fixed to a substrate by a braze, has a shape to facilitate escape of voids from between the pin and the substrate during the time the braze is in a liquid state.

[0005] Preferably, the shape is such as to allow buoyancy forces acting within the braze to cause voids to escape from between the pin and the substrate.

[0006] Advantageously, the one end has a substantial portion of its surface at an angle with respect to a plane perpendicular to the axis of the pin.

[0007] From another aspect, the invention provides a method of forming a brazed joint between a pin and a substrate, characterised in that the pin has one end with a shape to facilitate escape of voids from betwen the pin and the substrate during the time the braze is in a liquid state, that the shaped one end of the pin is held in position relative to the substrate with the braze in molten state, and that-the braze is permitted to cool from the molten state for a period of time sufficient to allow the escape of voids in the braze between the substrate and pin.

[0008] In a preferred embodiment of the invention, the pin has a sloped (e.g. conical or wedge shaped) head at the end that will be brazed to a module. The slope at the pin head is sufficient to permit bubbles trapped in the braze to "float" out of the braze while it is in its liquid state.

[0009] The primary advantage of this invention is that its use will reduce voids in the brazed joint between a pin and a module, resulting in a structure of increased mechanical durability.

[0010] A secondary advantage of the invention is that the shape of the pin is such that it can be butted directly against the substrate during brazing, thus obviating the need to control the spacing between the pin head and the module.

[0011] Another advantage is that the pin may be used without significantly impacting the manufacturing process by which modules are produced.

[0012] The scope of the invention is defined by the appended claims; and how it can be carried into effect is hereinafter particularly described with reference to the accompanying drawings, in which

Figure 1 is a schematic view of a brazed joint between a solid state module and a prior art pin;

Figure 2 is a sectional view, to an enlarged scale, through the braze between the prior art pin and module of Fig.l;

Figure 3 is a view, similar to Fig.1, of a preferred embodiment of this invention; and

Figure 4 is an enlarged diagrammatic detail of the preferred embodiment showing the forces on a void.

[0013] Figure 1 shows a prior art pin 1 that is brazed to a substrate 2 whose surface is in a plane perpendicular to the axis of the pin, by braze material 3. The purpose of the pin may be to provide an electrical contact between a pad (not shown) on the surface of the substrate and other parts of an electrical system, or it may be used to provide mechanical contact (e.g. for mounting purposes) between the substrate 2 and other parts of a larger system, or a combination of the two. The pin typically is made of Kovar and has a flat pin head about 0.4 mm in diameter. The space between the flat head of the pin 1 and the substrate 2 is typically about 0.07 mm, and is filled by the braze material, which is typically a combination of gold and tin. Where the braze material contacts the surface of substrate 2, the surface of the substrate must comprise a material that is wettable by the braze.

[0014] In the braze material 3 below the head of the pin 1, whose area is indicated by the broken line 4 in Fig.2, is a number of voids 5. These voids (which are also shown in Fig.l) are the result of gas bubbles which were trapped under the pin head when the braze was in its molten state. These voids represent a significant source of potential failure of the mechanical joint between the pin and the substrate.

[0015] In a preferred embodiment of the invention (Fig.3) a pin 6 is brazed to a substrate 2 by braze material 3. The pin 6 has a head at the end which will be brazed to the substrate, which, instead of being flat as in the prior art, is conical in shape. The apex of the cone is in direct contact with substrate 2. The inclined sides 7 of the cone are preferably long enough to extend beyond the area of the body of pin 6 so that the total volume between the head of pin 6 and the substrate 2 will be comparable to the volume between the flat head of pin 1 and substrate 2 shown in Fig.l. This is desirable in order to have a sufficient volume of braze material between the pin head and the substrate. Except for this head, the pin 6 is preferably identical in size to the prior art pin 1 shown in Fig.l.

[0016] When the braze is in its molten state, voids formed as a result of gas bubbles will not be trapped under the conical pin head but will move out from between the pin head and the substrate.

[0017] Fig.4 illustrates the forces that act upon a void 8 that is in the braze (for enhanced clarity, the braze is not shown in Fig.4) between the head of the pin 6 and the substrate 2. While the braze is in its molten state, there will be a buoyancy force F that is pushing up on the void 8. That force can be regarded as having two components, F_N normal to the inclined surface of the head of the pin 6 and F_p parallel to the inclined surface of the head of the pin 6. While the braze is in its molten state this latter component of force will tend to push the void 8 along the surface of pin 6 and out from under the pin head. The buoyancy forces on the void will cause it to move out at a velocity:

where:

g = acceleration due to gravity, 980 cm/sec

r = radius void

p = density of the braze

µ= viscosity of the molten braze

θ = the angle between the head of the pin and the substrate

[0018] Thus, the radius r min of the smallest void which is certain to escape from between the head of the pin and the substrate in a given amount of time t is :

where S = the distance from the apex of the pin head to the outer extremity of the pin head.

[0019] Given a system in which the distance S is 0.5 mm, the angle 9 = 15°, the braze is an 80/20 mixture of Au/Sn (µ = 3 0.05 poise and p = 15 g/cm ) and t = 10 minutes (the approximate time during which the braze will remain molten after having been heated to 280°C) then r_min = 1 pm. This means that one can be fairly certain that substantially all voids having a radius larger than 1 micron will escape from between the pin head and the substrate. (For example, a void having a radius of 2 microns would take about 2.5 minutes to escape if it started at the apex of the pin head.) Of course, even smaller voids will escape if they start away from the apex.

[0020] The pin head need not necessarily be conical in shape. For example, it could be shaped like a wedge, a pyramid or a chisel or even be spherical in shape. It could also have a small portion of its head flattened if a larger area of contact between the pin and the substrate were desired.

[0021] Another variation would be to shape the pin head in such a manner that the angle between it and the substrate varies. The greatest angle could be used in portions where voids are likely to be most detrimental. The larger angle would cause voids to move away most quickly in this area.

[0022] In yet another variation of the invention, the head of the pin need not be in direct contact with the substrate. However, it is preferred that direct contact be made because this will generally simplify the manufacturing process by eliminating any need to maintain a specific distance between the pin head and the substrate.

[0023] The primary and most essential part of this invention is that a very substantial portion of the surface of the pin head that faces the substrate be at an angle to the substrate so that buoyancy forces within the molten braze will cause voids to move out from between the pin head and the substrate.

[0024] The most significant advantage of this invention is that its use will reduce the number and total volume of voids in the braze material between a pin head and a substrate, thus increasing the mechanical strength of the braze and making it less prone to failure.

[0025] Another advantage of the preferred embodiment of the invention is that, because the pin head is placed in direct contact with the substrate, there are no critical spaces to maintain between the pin and the substrate.

[0026] Still another advantage of the preferred embodiment of the invention, which further enhances the mechanical strength of the braze, is that the pin head itself, because it goes completely through the braze material and contacts the substrate, can form a mechanical barrier to propagation of some cracks within the braze. Cracks which in the prior art would have propagated completely through the braze between the pin and the substrate may be intercepted by the pin itself and thus be prevented from propagating through the braze.

[0027] Another significant advantage to this invention is its simplicity of implementation. No substantial changes to an existing process will generally be required, except for a change in the shaping of the pin head.

[0028] The angle θ and length S of the pin head will depend upon the braze that is used and the desired minimum radius of voids for which escape is to be ensured. The specific details of implementation of the invention into any existing manufacturing process are well within the knowledge of those skilled in the art in light of the above detailed description.

Claims

1 A pin, one end of which is to be fixed to a substrate (2) by a braze (3), characterised in that the one end of the pin has a shape to facilitate escape of voids from between the pin and the substrate during the time the braze is in a liquid state.

2 A pin according to claim 1, in which the shape is such as to allow buoyancy forces acting within the braze to cause voids to escape from between the pin and the substrate.

3 A pin according to claim 1 or 2 in which the one end has a substantial portion of its surface at an angle with respect to a plane perpendicular to the axis of the pin.

4 A pin according to claim 3, in which the one end is tapered.

5 A pin according to claim 3, in which the one end has a substantially conical shape.

6 A pin according to claim 3, in which the one end is substantially wedge shaped.

7 A pin according to claim 3, 4, 5 or 6, in which the angle is equal to or larger than an angle 6 which satisfies the relationship:

where:

r = the radius of the smallest void for which escape is desired to be reasonably certain,

µ = viscosity of the molten braze,

p = density of the braze,

g = acceleration due to gravity,

t = time during which the braze will remain in a molten state, and

S = maximum distance of travel by a void in order for it to escape from the braze between the end of the pin and the substrate.

8 A brazed joint between a substrate and a pin according to any preceding claim, in which the shaped one end of the pin is brazed to the substrate by braze material.

9 A brazed joint according to claim 8, in which the shaped one end of the pin is in direct contact with the substrate.

10 A method of forming a brazed joint between a pin and a substrate, characterised in that the pin (6) has one end with a shape (7) to facilitate escape of voids from between the pin and the substrate (2) during the time the braze (3) is in a liquid state, that the shaped one end of the pin is held in position relative to the substrate with the braze in molten state, and that the braze is permitted to cool from the molten state for a period of time sufficient to allow the escape of voids in the braze between the substrate and pin.

11 A method according to claim 10, in which the shaped one end of the pin is in direct contact with the substrate.

Drawing